Optimization Of Mixing For Silica Fume Concretes

Silica fume is a highly effective pozzolanic material due to its extreme fineness and high silica content. Silica fume have been used in concrete to improve its properties. It has been found that silica fume improves compressive strength, bond strength, frost resistance and abrasion resistance; reduces permeability and therefore helps in protecting reinforcing steel from corrosion. Silica fume concrete has been extensively used in regulations of water resources and hydropower engineering, construction engineering, highway engineering and bridge engineering. At present, regulations of water resources engineering in freezing regions in China need great deal of concrete which is permanent stability and frost resistance. Silica fume is used in concrete to improve permanent stability and frost resistance. Silica fume concrete is suitable for freezing regions. Silica fume concrete will be more extensively used in regulations of water resources and highway engineering in cold regions. Research on mix proportions of silica fume concrete has important theoretical meaning and can provide a basis for applications.When silica fume concrete is used, mix design was made. At the same time, we should choose reasonably binder, sand, and coarse aggregate. In this paper, results of this comprehensive experimental investigation were analyzed for compressive strength, tensile strength, ultrasonic pulse velocity, permeability and frost resistance. The main objective of this investigation is determine the different effects silica fume has when added a partial substitute to Portland cement. The main contests and conclusions are listed as follows:1. The compressive strengths are measured on three different types of specimens. The compressive strength of 150mm cubes standard specimen is lower than the compressive strength of 100mm cubes specimen. The compressive strength of 100mm×200mm cylinder specimen is lower than the standard specimen.Extensive experimentation is performed to determine the isolated effect of silica fume on the properties of concrete over a wide range of water-to-binder ratios. On the basis of analysis of a large number of experimental results, silica fume incorporation can highly increase the compressive strength of concrete in the range of 0.2-0.45 water-to-binder ratio when silica fume content is beyond 25％. The compressive strength of concrete reduces with the increase of silica fume content over 25％.2. Extensive experimentation is performed to research the effect of silica fume content and water-to-binder ratio on the split tensile strength of concrete. Silica fume content incorporation increases the split tensile strength of concrete. Very high percentages of silica fume do not significantly increase the split tensile strength,. Silica fume incorporation also increases the flexural tensile strength of concrete. But the flexural strength will reduce if silica fume content is too much.3. Based on tests results, water penetration of silica fume concrete is investigated. Silica fume incorporation can increase the water penetration of concrete. At the beginning, water penetration increases with the increases of silica fume content. Finally, water penetration reduces with the increases of silica fume content. The optimum silica fume replacement percentage for water penetration is .suitable4. Nondestructive testing is also performed to evaluate performance of concrete mixtures. The increased silica fume content results in reduced ultrasonic pulse velocity. On the other hand the increase in water content has led to an equivalent decrease in velocity values. In general, water-to-binder ratio of 0.2 showed the highest values of velocity. A model defining the relation between the ultrasonic pulse velocity and different factors affecting the performance of concrete is developed.According to compressive strength and ultrasonic pulse velocity test results, Correlation between pulse velocity and compressive strength showed different trends depending on silica fume content. For all silica fume levels, nonlinear quadratic models describing the relation between pulse velocity and compressive strength are obtained. The formula is very exact, and the Maximum error is acceptable.5. Drying shrinkage increases with silica fume content up to 10% replacement level. However, with 15% silica fume the drying shrinkage registered at 28 day shows a different trend. The increase in shrinkage strain is major in the first week, and then it was almost stabilized until the age of 28 days.In the range of 0.2-0.25 W/C, weight loss of silica fume concrete is lower than plain concrete. If water-binder-ratio is beyond 0.3, weight loss of silica fume concrete is higher than plain concrete. The maximum weight loss is obtained at a silica fume replacement of 10％. Less weight loss was observed for 15％silica fume concrete.6. Antifreeze and superplasticzer incorporation can increase frost resistance of concrete. Compare to silica fume and water-binder-ratio, the effect of Antifreeze and superplasticzer on frost resistance of concrete is very little. Silica fume incorporation increases highly frost resistance of concrete. The optimum silica fume replacement percentage for s frost resistance is 10％. If silica fume is 10％, index of frost resistance DF is larger than 0.4. If water-binder-ratio is from 0.3 to 0.35, index of frost resistance DF is larger than 0.6. At the beginning, frost resistance of concrete increases with the increase of silica fume. Then, frost resistance of concrete reduces with the increase of silica fume.